the manufacturing process of semiconductor devices requires defining and forming, one inside the other, active regions which differ for the type of dopant ("p" or "n") or for its concentration.
It is often necessary that these regions occupy the smallest possible space, with the purpose of:
allowing a higher degree of integrations; and
improving the performance of the device in connection with the reduction of the resistances associated with the electric paths of the current.
Wishing to define a type n doped region within a larger type p doped region, using the photolithographic techniques most commonly used for the definition of the regions, would limit the minimum realizable dimensions at the following values:
implanting by projection litography: 4 .mu.m for region p, 2 .mu.m for region n;
implanting by "stepper" type photoexposure: 1.7 .mu.m for region p, 1 .mu.m for region n.
To define and form these regions without the above mentioned dimensional limits imposed by photolithographic techniques, such it is necessary to adopt self-alignment techniques, as the formation of "spacers", which, however, are only applicable for the definition of two "long" active regions, one inside the other (hereinafter the expressions "long region" and "short region" will be used to indicate regions whose sizes are of different orders of magnitude).
In fact, to define a "short" region inside a "long" region it is absolutely necessary, according to known techniques, to adopt the above mentioned photolithograhic techniques, with the minimum dimensional limits they impose.